Earlier quoted context omitted.
Your comments are a public thing you choose to publish to the world, but many (most?) discord messages are not. You'd want them to get deleted.
Sorry but no, I don't want my messages deleted. I use search history all the time even to search for things I personally said. I can get behind deleting messages if the account is deleted though, but as some type of automatic thing based on time in the past? No thanks.
Nobody is making the argument you should be forced to delete your messages.
In any normal world, messages that are not used would be deleted as a matter of privacy. They're kept, because they can be kept, and they can be monetized. That monetization has zero benefit to the user, it's just an artifact of our odd way of doing business where we continue to externalize a lot of things. I think over the next 10 years we might see a regulator shift , which also means costs more directly exposed, meaning Discord may cost $1 month, i.e. the externalization 'costed in' like carbon tax on fuels.